This invention relates to a control device for a sealed, oil insulated electric motor installed in a deep-sea submarine or the like, and particularly to a control device for maintaining the oil at a predetermined temperature during idle periods to thereby reduce the startup torque and current drain due to the otherwise viscous nature of the cold oil.
Generally, in deep-sea work boats, research vessels or the like which submerge to depths of thousands of meters, various electric motors are installed for use in the propulsion of the main and auxiliary engines, sea-water pumps, hydraulic pumps, etc.
FIG. 1 illustrates one such motor, configured as a three-phase AC induction motor 4, having a primary winding 11 wound around a stator core 42 provided on the inner periphery of a frame 41. Oil lubricated ball or roller bearings 43 are mounted at the ends of the motor to rotatably support a shaft 44 mounting a rotor core 45. A shaft seal 46 is installed at one end of the motor. Installed at the other end of the motor is an equalizing device 47 for equalizing the pressure inside the motor 4 to the outside pressure of the surrounding sea water upon submergence. The equalizing device 47 comprises an expandable bellows 48, a tension spring 49, and a support plate 50. The motor frame 41 is filled and sealed with oil 51, which is both electrically insulating and lubricating.
Such oil has increasing viscosity and deteriorating fluidity with a decrease in temperature or an increase in pressure.
When a vessel submerges the pressure of the sea water is applied to the interior A of the bellows 48, and the pressure of the oil 51 inside the motor 4 can thus mount to hundreds of atmospheres at a temperature of about 1.degree.-2.degree. C. when the motor is not operating (i.e., cold state).
When the vessel submerges to great depths the oil inside the idle motor is thus at a low temperature and under high pressure, and as a result the motor is difficult to start again because of the large opposition torque due to the high viscosity of the oil. To operate under such conditions it is necessary to employ an unduly large motor having a high starting torque capable of overcoming such factors as the static frictional force of the machine to be driven, the high pressure due to the depth of submergence, and the viscous resistance of the low temperature oil. The motor output is thus excessive under normal or running conditions.